Project summary: Autoimmunity occurs when the body's immune system mistakenly attacks normal tissues, leading to disease. Treatments for autoimmunity chronically suppress the immune system, which risks fatal infection; thus the ideal therapy would eliminate only the disease-causing autoimmune cells while preserving normal immunity. We recently developed a novel gene-engineered chimeric autoantibody receptor T cell (CAAR-T) therapy that uses the autoantigen targeted in disease to direct T cell cytotoxicity against only those B cells that express autoantigen-specific B cell receptors (BCRs). We have shown that CAAR-Ts cause complete histologic and serologic remission of autoantibody (autoAb)-mediated disease in an experimental mouse model and engraft to form memory CAAR-Ts that can provide lasting protection against disease recurrence. In considering how to translate CAAR-T technology to first-in-human trials, a challenge arises given that most autoimmune patients do not face imminent death from their disease, unlike the first cancer patients treated with gene-engineered cellular therapies. The current preclinical pipeline relies on mice whose disease is artificially induced and treated shortly after disease induction. Cytokine release syndrome does not occur in mice despite being one of the most common toxicities in cancer patients. The field has not pursued better preclinical models given the risks of time and money involved in experimenting in a new animal species. Given the curative potential for CAAR-T in autoimmunity and the increasing interest in developing cellular therapies for non-oncologic indications, we believe it is now crucial to establish a higher preclinical standard. We thus seek to establish dogs with naturally occurring autoimmune disease as an ideal preclinical system for assessing cellular immunotherapies, which we believe will better predict the safety and efficacy of human therapies than experimental mouse models. This proposal meets the high-risk, high-reward nature of the transformative R01 program, as gene-engineered cellular therapies have never been tested in any naturally occurring autoimmune disease. The successful treatment of autoimmunity in dogs with CAAR-T would not only be a breakthrough in veterinary medicine, but would also establish dogs as an ideal species for preclinical validation of human cellular immunotherapies and provide compelling evidence for doctors and patients to enroll for future CAAR-T first-in-human trials. Ultimately, our work could facilitate the translation of cellular immunotherapies for a broad range of canine and human diseases.